In the prior art it was known to incorporate within NMR data control systems a FIFO containing a sequence of codes defining the respective sequence of operative states of the apparatus, each together with a persistence datum encoding a corresponding time interval during which each state is maintained. The imminent underflow of the FIFO caused an interrupt to a host processor which responded to reinitialize the FIFO. The sequence was therefore executed in precise synchrony. Such a system is described in U.S. Pat. No. 4,191,919.
Further, the FIFO based systems of the prior art address system requirements for enhanced information density by implementing re-entrant operation of the FIFO to reduce interrupt overhead. In this manner an entire set of periodically repetitive operations contained in a re-entrant FIFO structure could be rotated through an output register for a desired number of cycles (of the sequence) to control the spectrometer accordingly and re-inserted in the FIFO to maintain the queue. Another prior art feature permitted compressed repetitive instructions by defining a repetition field of the FIFO word for the purpose of specifying the number of times that a particular state and persistence interval are to be repeated. The purpose of such FIFO based system enhancement is to effectively increase the effective FIFO capacity in the number of control words asserted without interrupt of the host processor. A reentrant FIFO based system also incorporating a repetition feature is described in U.S. Pat. No. 4,375,676.
These prior art FIFO based systems are well adapted to many repetitions of a sequence of words not exceeding the maximum FIFO capacity in length, or to a large number of repetitions of a particular control word. These techniques are not well adapted to the execution of compact macros of instructions which may be desired for execution within the periodic sequence or to very long (non-repetitive) sequences which exceed the FIFO capacity.